The need to reduce vehicle weight to achieve better fuel economy has lead to an increase in the use of aluminum in the structural and exterior components of vehicles. As a result, new processes must be developed which reflect the different material properties of aluminum and which will allow aluminum parts to replace steel in various applications.
For example, when two pieces of metal are joined on the exterior of the vehicle, the joint must be disguised so that it is not apparent that there is more than one piece of metal. The so-called seamless joint aluminum body panels are currently processed by welding and then grinding the joint. A Class-A finish can be produced by grinding the weld bead down to produce a smooth paintable surface. This results in significant thinning of the base material. Measurements have shown removal of more than 50 percent of the material, which has serious implications for the integrity of the panel. The material's integrity must be maintained after grinding, or distortions may appear at the joint after the panel is processed through the paint ovens.
One way to avoid the potential for sinking/distortion of the joint is to add extra material in the form of spray filler that produces a paintable Class-A finish. The term thermal spray process describes a group of well-known processes for depositing metallic, non-metallic, and mixed metallic/nonmetallic coatings. These processes all require a heat source, a propelling device, and a feed material. The processes include flame spraying (including combustion flame spray, and high-velocity oxy-fuel (HVOF) thermal spray devices, plasma spraying (including powder plasma spraying, and plasma transferred wire arc deposition), electric arc spraying (including twin wire arc spraying), and detonation spray.
A flame spray device typically deposits metals or ceramics on a substrate. The flame spray device includes a combustion chamber which receives a mixture of fuel and oxidant as a pressurized gas, and creates a combustion reaction in a high pressure, high temperature stream. The flame spray device directs the combustion stream from the combustion chamber to a flow nozzle. The spray material enters the high velocity combustion stream, which melts the spray material at least partially. The combustion stream atomizes the partially (or completely) melted spray material, and sends it toward the surface of the substrate.
A plasma spray device generates and sends out a high velocity, high temperature gas plasma which delivers a powdered or particulate material to the surface of the substrate. The plasma spray device forms the gas plasma by sending a gas through an electric arc in the nozzle of a spray gun, causing the gas to ionize into the plasma stream. The spray material, which can be preheated if desired, is introduced into the plasma stream and directed to the surface of the substrate.
An arc spray device generates an electric arc zone between two consumable wire electrodes. As the electrodes melt, the arc spray device feeds the electrode wires into the arc zone. A compressed gas is delivered into the arc zone where it atomizes the molten surfacing material and propels it to the surface of the substrate.
The arc spray process is currently used in the industry to provide extra material for large steel joints. Silicon bronze is heated and deposited on the roughened surface of the weld component and then sanded to a smooth finish.
However, these thermal spray techniques have not been used on aluminum body panel joints to help achieve a Class-A finish.
New joining methods must be developed that permit aluminum to be used in the difficult joint designs demanded by vehicle styling. A process is needed to produce a Class-A surface when aluminum parts are welded together. The process should allow production of a smooth paintable surface without significant thinning of the base aluminum.